The following events must be subscribed in the constructor so the placeholder could be toggled according to the text box property “Text”.

public TextBoxNumbers()
{
InitializeComponent();
// No one of theese events will start, because
// TextBox control is still in construction
this.TextChanged += new EventHandler(this.PlaceHolder_Toggle);
this.LostFocus += new EventHandler(this.PlaceHolder_Toggle);
this.FontChanged += new EventHandler(this.PlaceHolder_Toggle);
this.GotFocus += new EventHandler(this.PlaceHolder_Toggle);
this.LostFocus += new EventHandler(this.PlaceHolder_Toggle);
}